1. Field of the Invention
The present invention relates to an optical transmission system and its related techniques. More particularly, the invention relates to an optical transmission system having a transmission line with its transmission conditions optimized for large-capacity transmission, an optical transmission system employing an optical multiplexing technique such as optical time-division multiplexing (ODTM) to achieve large-capacity transmission, and related techniques for implementing the same.
2. Description of the Related Art
Increasing the transmission speed severely limits the transmission distance because of the waveform distortion, caused by group-velocity dispersion (GVD), in optical fibers. Furthermore, when the transmission speed is increased, the optical power for transmission needs to be increased to maintain required difference between transmitted and received optical power levels. This in turn increases the effect of self-phase modulation (SPM), a nonlinear effect of optical fibers, which further complicates the waveform degradation through interaction with group velocity dispersion (SPM-GVD effect).
When the waveform distortion caused by the SPM-GVD effect is dominant, the scaling rule expressed by the following equation essentially holds.
DB.sup.2 P.sub.av L.sup.2 =const. (1) PA1 D: dispersion value (ps/nm/km) PA1 B: transmission rate (Gb/s) PA1 P.sub.av : average optical power through transmission line (mW) PA1 L: transmission distance (km) const.: determined by required penalty
For example, when the transmission rate B is quadrupled from 10 Gb/s to 40 Gb/s, the average optical power P.sub.av through the transmission line needs to be quadrupled. This means that to achieve the same transmission distance, the dispersion value D at signal wavelength must be set to 1/64.
To minimize the dispersion value of signal light, work is currently under way to transmit signals in the 1.55-.mu.m range by using a dispersion-shifted fiber (DSF), an optical fiber whose zero-dispersion wavelength .lambda..sub.0 is shifted to the 1.55-.mu.m range where fiber transmission loss is minimum. The zero-dispersion wavelength .lambda..sub.0 is the wavelength at which the chromatic dispersion value D (ps/nm/km), representing the amount of change of propagation delay time with respect to slight variations in wavelength, changes from negative (normal dispersion) to positive (abnormal dispersion). Near this wavelength .lambda..sub.0, the absolute value of chromatic dispersion becomes the smallest, so that the waveform distortion due to the chromatic dispersion is reduced to a minimum.
However, since the fiber drawing process introduces slight variations in fiber core diameter, the zero-dispersion wavelength .lambda..sub.0 of a DSF transmission line is inevitably subjected to variations along its longitudinal direction. Furthermore, transmission cables are fabricated by connecting segments of multi-core cables, each segment extending several kilometers, and the wavelengths .lambda..sub.0 between adjacent segments are not continuous but randomly distributed. Moreover, .lambda..sub.0 varies with aging and due to changes in ambient temperature, etc.
Therefore, in the prior art, the worst-case design has been employed by which the transmission line has been designed by considering the distribution of .lambda..sub.0 and the deterioration with time so that the required transmission quality can be satisfied even if the worst-case value is applied throughout the transmission line. This has inevitably increased transmission line costs, which has impeded the implementation of high-capacity transmission systems.
On the other hand, signal processing, such as modulation and demodulation of optical signals, is usually performed at the electrical signal level, and it has been standard practice to increase the speed of optical transmission systems by increasing the speed of electrical signals used to modulate optical signals. In recent years, however, increasing the speed at the electrical signal level using electronic devices has been posing a difficult problem. Research and development is being undertaken on optical communication devices, at 10 to 40 Gb/s, using Si, GaAs, HBT, HEMT, etc., but it is said that at the present state of technology, 10 to 20 Gb/s is the maximum for practical implementation.
Therefore, to increase the transmission speed of optical transmission systems beyond the operating speeds of electronic devices, multiplexing techniques in the optical region provide effective means. There are two main techniques that can be used: one is wavelength-division multiplexing (WDM) and the other is optical time-division multiplexing (ODTM). For practical implementation of either technique, development of related peripheral techniques is needed.